The present invention relates to the field of high temperature annealing of steel coils.
In the manufacture of flat-rolled steel sheet and strip products, it may often be important to intermittently anneal the material for further cold-rolling operations. It may also be necessary to anneal the material at the finish gauge to render it suitable for fabrication (e.g., stamping and cold forming). Annealing is important because cold reduction elongates the grains of the steel microstructure, distorts the crystal lattice, and induces internal stresses. The steel that results from the cold reduction process is typically very hard and has reduced ductility. The annealing process recrystallizes the cold-worked steel, and if the steel is held at the proper annealing temperature for a sufficient time, the microstructure of the annealed steel will return to substantially undistorted lattices and the steel will be more ductile. Cold-rolled steel and heat-resisting steel sheets can be produced by hot rolling, hot annealing and pickling, cold rolling-finish annealing and pickling (cold-rolled annealing and pickling), and subsequent skin-pass rolling. The finish annealing and pickling procedure generally comprises a continuous annealing, pickling or continuous bright annealing.
Annealing techniques may be divided into two general categories: (a) batch operations, such as conventional box annealing; and (b) continuous operations. In the steelmaking, the softening of flat rolled sheet and strip products is typically accomplished through the use of continuous annealing.
The continuous annealing process involves unwinding the coil from a payoff reel, continuously feeding the coil through an annealing furnace, and then rewinding the coil on a take-up reel. The annealing furnace is typically electric or gas fired. The steel strip, while traveling through the furnace, is typically heated to a temperature in the range of about 1800° F. to about 2200° F. in the case of austenitic alloys, and to a temperature in the range of about 1400° F. to about 1800° F. for ferritic alloys. The annealing temperatures vary depending upon the particular alloy being annealed and desired microstructure, as well as the end-use of the steel alloy.
A continuous annealing line is useful for mass production, but may not always appropriate for smaller production runs. Instead of finish annealing (cold-roll annealing) and pickling requiring huge facilities, the use of box annealing (also called “bell annealing” or “batch annealing”) may be of economically advantageous in shorter production runs.
Batch coil annealing furnaces (sometimes called “box annealing furnaces” or “bell shaped” furnaces) have been long used and are well known in the industry. In such furnaces steel coils are stacked vertically, edge to edge, on a base, and a removable inner cover is placed over the stacked coils. An outer cover then is placed over the inner cover, and the covers are removably sealed to the base. The outer cover typically contains gas fired burners that heat the inner cover, and in turn radiation heats the stacked coils. Batch coil annealing processes in the steel mill industry typically take about 20 hours to several days to complete.
Prior art annealing furnaces are off-line annealing furnaces. Because of the size and time requirements of annealing, the furnaces are usually not in-line in a steelmaking line. Annealing treatment of metal product in such off-line processes normally involves long cycle times, resulting in low productivity levels, high heat treatment costs and less energy savings.